When drops of water are sprinkled on an extremely hot skillet, the drops can slide around the skillet for a full minute or so before evaporating. The phenomenon is called the Leidenfrost effect, which says that when a surface is significantly hotter than the boiling point of a given liquid (the Leidenfrost point), droplets of that liquid will take longer to evaporate than if the temperature of the surface were somewhat cooler - above the liquids boiling point but below the Leidenfrost point. (For water, the Leidenfrost point is 250 °C [482 °F].)

The effect occurs because the bottom part of the droplet immediately vaporizes on contact with the hot surface, and the resulting gas suspends the droplet above the surface. No longer touching the hot surface, the droplet evaporates more slowly and, due to reduced friction, can slide around on the thin layer of gas.

In a new study to be published in PNAS, Marie Le Merrer from CNRS-Ecole Polytechnique in Palaiseau, France, and CNRS-ESPCI-Paris, and her coauthors have investigated the cause of the deceleration of liquid nitrogen drops sliding in a Leidenfrost state just above a liquid surface. Unlike typical Leidenfrost drops on hot surfaces, these Leidenfrost drops exist at room temperature.

A liquid nitrogen drop (3 mm in diameter) decelerates on water, its velocity decreasing from 60 cm/s to 15 cm/s. The movie is slowed down 20 times. Two wakes can be noticed: a wake in the air that is due to vapor condensation, and a wake from the waves at the surface of the water, created at the front of the drop. Credit: Marie Le Merrer, et al.

In their experiments, the researchers carefully threw millimeter-sized liquid nitrogen drops at a certain velocity tangent to the surface of water or silicone oil. Then they recorded the drops motion with a high-speed video camera from above. Although the drops took about a minute to evaporate, the researchers were interested only in the first second of the drops motion. They found that the drops on water slowed down significantly between 0.05 and 0.18 seconds, with a deceleration of about 170 cm/s2.

This deceleration of drops on water is much higher than it is on a solid surface (where deceleration is about 10 cm/s2), suggesting that the friction on water is about 10-100 times stronger than the friction on a solid surface. When analyzing their video, the researchers found that the increased friction on water is due primarily to wave resistance. A drop moving above the waters surface can generate waves, whose wake the researchers observed in the videos.

A liquid nitrogen drop on water (top) and on glycerol (bottom), which has a viscosity that is 1,000 times higher than that of water. The movie is slowed down 40 times. The drops have the same initial velocity (60 cm/s). The drop on glycerol "wins the race" and arrives earlier than the drop on water, showing that the deceleration is lower on the more viscous glycerol. Credit: Marie Le Merrer, et al.

When performing the same experiment on the more viscous oil, the researchers found that the deceleration is much lower than it is on the water, and they did not observe waves as they did on the water. These results further support the idea that wave resistance is the main contributor to the deceleration of Leidenfrost drops on water.

The study helps to clarify wave resistance, which occurs in many situations but is usually difficult to measure since friction on an object moving at the waters surface is the sum of different contributions, from which it is difficult to extract the sole wave resistance. By using floating bodies that do not penetrate water or oil, the researchers could ensure that the main source of resistance was wave resistance. Understanding wave resistance could have a wide variety of implications, such as providing a clearer picture of the motion of insects at the surface of water ponds.

Related Stories

(PhysOrg.com) -- A Swinburne University professor was part of a team that showed that drag on hot bodies moving through liquid can be radically reduced by up to 85 per cent, potentially doubling their speed.

A recent experiment conducted by physicists at University of Bristol in the United Kingdom has shown that liquid drops can defy gravity. Droplets of liquid on an inclined plate that is shaken up and down can travel uphill ...

To the gardening world it may have always been considered a fact, but science has never proved the widely held belief that watering your garden in the midday sun can lead to burnt plants. Now a study into sunlit water droplets, ...

(PhysOrg.com) -- Hiroaki Katsuragi and a team from Kyushu University in Fukuoka, Japan, have been investigating what happens when water drops of various sizes are allowed to fall from a height of 10 to 480 mm onto a granular ...

Recommended for you

(Phys.org)—Physicists have built one of the first basic elements of a trapped Rydberg ion quantum computer: a single-qubit Rydberg gate. The achievement illustrates the feasibility of building this new type of quantum computer, ...

Researchers have demonstrated prototype windows that switch from reflective to clear with the simple addition of a liquid. The new switchable windows are easy to manufacture and could one day keep parked cars cool in the ...

For anyone who has marveled at the richly colored layers in a cafe latte, you're not alone. Princeton researchers, likewise intrigued, have now revealed how this tiered structure develops when espresso is poured into hot ...

Vector polarizers are a light filtering technology hidden behind the operation of many optical systems. They can be found, for instance, in sunglasses, LCD screens, microscopes, microprocessors, laser machining and more. ...

Depending on the dose and the target, radiation can cause incredible damage to healthy cells or it can be used to treat cancer and other diseases. To understand how cells respond to different doses of radiation, scientists ...

If you watch flat surfaces in the rain (like a car hood) carefully spherical droplets form and do the same thing for a bit.

If you watch a pond when there is a little wind you will occasionally see little V wakes run on the surface for a few inches. I wondered what caused that for a while thinking it was either a bug or something under the water. It wasn't either of those. I think it is the same phenomena, a small bead of water blown by the wind and leaving a wake.

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.